Comparison of red deer and possum diets and impacts in podocarp-hardwood forest, Waihaha Catchment, Pureora Conservation Park SCIENCE FOR CONSERVATION: 50 G. Nugent, K.W. Fraser, and P.J. Sweetapple Published by Department of Conservation P.O. Box 10-420 Wellington, New Zealand 1 Science for Conservation presents the results of investigations contracted to science providers outside the Department of Conservation. Reports are subject to peer review within and outside the Department. © June 1997, Department of Conservation ISSN 1173-2946 ISBN 0-478-01898-3 This publication originated from work done under Department of Conservation contract carried out by G. Nugent, K.W. Fraser and P.J. Sweetapple, Landcare Research, P.O. Box 69, Lincoln. It was approved for publication by the Director, Science and Research Division, Department of Conservation, Wellington. Cataloguing-in-Publication data Nugent, Graham. Comparison of red deer and possum diets and impacts in podocarp-hardwood forest, Waihaha Catchment, Pureora Conservation Park / G. Nugent, K.W. Fraser, and P.J. Sweetapple. Wellington, N.Z. : Dept of Conservation, 1997. 1 v. ; 30 cm. (Science for conservation, 1173-2946 ; 50.) Includes bibliographical references. ISBN 0478018983 1. Forest ecology- -New Zealand- -Pureora Forest Park. 2. Trichosurus vulpecula- -Feeding and feeds. 3. Red deer- -Feeding and feeds. 4. Pureora Forest Park (N.Z.) I. Fraser, K. W. (Kenneth Wayne) II. Sweetapple, P. J. (Peter James) III. Title. IV. Series: Science for conservation (Wellington, N.Z.) ; 50. 581.5223099338 20 zbn97-049815 2 CONTENTS Abstract 5 1. Introduction 6 2. Background 6 3. Objectives 8 4. Methods 8 4.1 Study area 8 4.2 Animal abundance and faecal output 8 4.2.1 Deer 9 4.2.2 Possums 10 4.3 Forest composition and forage availability 10 4.3.1 Over storey stem density and basal area 10 4.3.2 Litterfall production 10 4.3.3 Browse tier biomass and edible litterfall 11 4.3.4 Exclosures 11 4.4 Diet and forage consumption 11 4.4.1 Deer 11 4.4.2 Possums 12 4.5 Foliage preference and utilisation indices 12 5. Results 13 5.1 Animal abundance and faecal output 13 5.1.1 Deer 13 5.1.2 Possums 14 5.2 Forest composition and forage availability 15 5.2.1 Overstorey composition 15 5.2.2 Epiphytes 16 5.2.3 Litterfall 16 5.2.4 Edible litterfall 16 5.2.5 Browse tier biomass 17 5.3 Diet and forage consumption 18 5.3.1 Deer 18 5.3.2 Possums 20 5.3.3 Deer vs possums 21 5.4 Foliage preference and utilisation indices 22 5.5 Impacts on regeneration 24 5.5.1 Inhibition of height growth in woody plants 24 5.5.2 Exclosures 28 5.5.3 Relationship between animal density and seedling height 29 5.6 Summaries for key woody species 32 6. Conclusions 35 6.1 Animal abundance and faecal output 35 6.2 Diet and forage consumption 36 3 6.3 Impacts on regeneration 37 7. Recommendations 39 8. Acknowledgements 40 9. References 40 10 Appendix 10.1 Overstorey stem frequencies, stem densities and basal areas in the upper Waihaha Catchment 43 10.2 Epiphyte frequencies 45 10.3 Total annual litterfall biomass for 1991 and 1992 46 10.4 Biomass of potentially edible litterfall 48 10.5 Browse tier foliar biomass by species 49 10.6 Deer diet by season 51 10.7 Annual possum diet 53 10.8 Deer preference and utilisation indices 56 10.9 Possum preference and utilisation indices 59 4 Abstract The impacts of the two most widespread introduced herbivores in New Zealand (red deer and possums) were investigated to compare the relative importance of arboreal and terrestrial browsers, and to determine the likely relationships between pest density and their impacts. The study was conducted between 1990 and 1993 in podocarp-hardwood forest west of Lake Taupo, where the density, diet, and forage use of the two species was quantified and compared with quantitative estimates of forest composition, forage availability and production, and stem diameter and seedling height distributions. Deer density was stable at about c. 6 deer/km2, and annual faecal output for deer was estimated at 14 kg (dry weight)/ha/yr. Possum density increased during the study to reach c. 3 possums/ha in 1993, with an estimated faecal output of 44 kg/ha/yr for that year. In the forest, pepperwood (Pseudowintera colorata) and Neomyrtus pedunculata were most abundant, but podocarps comprised nearly half the total basal area of 73 m2/ha. About 2.5 tonnes/ha of foliage were produced annually, suggesting that c. 5 tonnes/ha of foliage were available to possums. In contrast, only 288 kg/ha of forage was available within the deer browse tier with just 9 kg/ha of that being comprised of the seven foods most important to deer. Deer and possums ate much the same range of about 100 plant species, but there was very little overlap in the main food used by each species. For deer, woody plants comprised 70% of the annual diet, ferns 17%, and grasses 10%, with broadleaf (Griselinia littoralis) and lancewood (Pseudopanax crassifolius) being the two most important foods. Most deer forage consisted of adult tree foliage, much of it obtained as litterfall. Woody plants also predominated (c. 80%) in possum diet, followed by herbs and ferns (c. 5% each). Possums relied heavily on fruit at times, but the foliage of Hall’s totara (Podocarpus hallii) was their main food. Overall, possums and deer consumed about 88 and 30 kg of forage/ha/yr, respectively, equating to 3.3% of total annual foliage production (AFP) for possums and just 1.1% of AFP for deer. Possums used 16 of the 40 most common species or species groups, but ate less than 12% of AFP for all but three of those. Deer consumed far greater proportions of the much smaller quantities of AFP available to them and, as a consequence, prevented regeneration for many of their preferred species. In contrast, possums appeared to have relatively little influence on the seedling heights (i.e., regeneration) of common species. Overall, there appeared to be little competition for food between the species, and possum impacts were focused on fewer woody plants than were deer impacts. Deer had a greater impact on regeneration patterns within the browse tier than did possums on the forest as a whole. Because neither species removed much of the total annual foliage production, major dieback or changes in abundance of the most common species appeared unlikely. However, most podocarps (excluding Hall’s totara) and other unpalatable species appeared likely to increase and many of the palatable species likely to decrease while deer and possums remained uncontrolled. A major reduction in deer density (i.e., to <2 deer/km2) would be required to significantly alter present regeneration patterns. If deer numbers remain above this level, intensive possum control would have little 5 effect on regeneration patterns, but should benefit established specimens of Hall’s totara and other possum-preferred species not accessible to deer. These results suggest the Department of Conservation should reassess present priorities in possum and ungulate (deer and goat) control for vegetation protection, giving greater consideration to the dominant influence of ungulates on patterns of ground-level regeneration. 1. Introduction The diets of red deer and possums and their likely impacts on the vegetation were assessed in a podocarp-hardwood forest by Manaaki Whenua-Landcare Research, Lincoln, for the Science and Research Directorate, Department of Conservation (DoC) as a complementary part of a Foundation for Research, Science and Technology (FRST)-funded investigation to quantitatively define and compare forage utilisation and impact patterns for the two species. The project was initiated in July 1989, with a planned 5-year time frame consisting of both pre- and post-control phases. However, deer and possum control was not implemented until winter 1994, so this report covers only the pre-control phase. 2. Background Red deer (Cervus elaphus scoticus) and possums (Trichosurus vulpecula) are the two most widespread introduced herbivores in New Zealand forests. Although they consume a similar range of plant species (Leathwick et al. 1983) there have been no direct studies of the dietary overlap between the two species. Our goal in this study was to determine the relative impacts of possums and deer on the native flora. We approached this by measuring deer and possum densities, diets, and forage consumption, and then relating the absolute amounts of forage consumed to the quantity available and the rate at which it was produced. From the inferred balances between forage production and animal consumption, we attempt to deduce which animal threatens which plant species and the likely nature of the various animal density - animal impact relationships. We address two main arguments: • The relationship between deer density and their impacts on regeneration of their food plants is non-linear, and this relationship differs between plant species. • Deer, rather than possums, have the greatest long-term impacts on forest composition and structure because these are ultimately determined mainly by ground-level regeneration patterns, not by the fate of canopy trees. 6 Edible litterfall (the wind-broken or cast, but still edible, green or yellowed foliage) from selected tree species has been identified as the primary source of deer and goat forage in native forests (Nugent & Challies 1988; Nugent 1990; Fraser 1991; Nugent 1993; Cochrane 1994). As a result, we hypothesise that the relationship between deer density and the regeneration of their food plants is unlikely to be linear (Nugent 1990; Nugent 1992, unpubl. FRI contract report).